High-purity potassium nitrate or potassium phosphate is used in horticultural or technical applications. Its purity should be above 97% based on dry matter.
The easiest way to produce high-purity potassium nitrate or potassium phosphate is to use a neutralization process involving potassium hydroxide and the corresponding mineral acid (nitric acid or phosphoric acid). This process is, however, not commonly used on an industrial scale, being reserved for special applications due to the high price of potassium hydroxide.
Ion exchange processes have therefore been developed for the production of high-purity potassium nitrate or potassium phosphate. These potassium salts are formed from an inorganic potassium salt, particularly potassium chloride, and the corresponding mineral acid (nitric acid or phosphoric acid) by means of an ion exchange process. For potassium nitrate, this is described in patent documents CA 2027064 (Monomeras) and U.S. Pat. No. 3,993,466, and for potassium phosphate in EP 230 355 (AST). Beside the valuable potassium salt, the ion exchange processes also produce an impure hydrochloric acid solution, which is often regarded as waste.
Organic material can also be a source of potassium. The potassium level of plants is generally about 1 g/100 g dry matter. However, this level can vary. For sugar beet it is 1 g/100 g, for potato 1.8 g/100 g and for grass 2.54 g/100 g dry matter. Potassium often accumulates in by-products during the processing of agricultural crops. For example, during starch extraction from potatoes the potassium level in the by-product (potato thick juice) can be as high as 14 g/100 g dry matter.
Agricultural by-products, for example molasses, can be used for industrial fermentation processes. After the valuable product, for example bakers yeast, ethanol, citric acid, has been extracted from the fermentation brot, a liquid with an increased level of potassium is obtained. This liquid, the fermentation by-product, is often concentrated and called vinasse. The potassium content can be as high as 14 g/100 g dry matter.
In addition to potassium the agricultural and fermentation by-products contain valuable organic compounds such as amino acids, proteins, organic acids and sugars, and are therefore in many cases used as an ingredient in animal feed. High potassium level is, however, undesirable in animal feed because it can lead to health problems (for example hypomagnesia in cows) and increases the production of manure.
There are many examples in the literature of the partial removal of potassium from agricultural or fermentation by-products in order to increase their value as animal feed ingredients. Different technologies are described, such as crystallization, chromatography, electrodialysis and ion exchange:
removal of potassium by crystallization is described in patent applications NL 6800310 and NL 6800313, corresponding to DE 1817550 and DE 1900242 (for molasses) and NL 9200403 (for potato thick juice); PA1 removal of potassium by chromatography is referred to in patent application WO 96/00776 (for vinasse); PA1 removal of potassium by electrodialysis is described in Int. Sugar Journal, vol. 95 (1993), pages 243-247 and PA1 removal of potassium by means of ion exchange is described in American Potato Journal, vol. 47 (1970), pages 326-336 (for potato juice). PA1 When hydrochloric acid is used, a mixture of potassium chloride and hydrochloric acid is produced. PA1 When sulphuric acid is used, a mixture of potassium sulphate and sulphuric acid is produced. PA1 Regeneration with nitric acid will yield a mixture of potassium nitrate and nitric acid. However, it was thought impossible to safely regenerate the resin with nitric acid in the presence of organic compounds. The main reason is the explosion hazard described in the literature for systems containing ion-exchange resin, organic compounds and nitric acid. PA1 Regeneration of the ion-exchange resin with phosphoric acid will yield a mixture of potassium phosphate and phosphoric acid. Phosphoric acid is, however, known to be a rather weak acid and so not very effective in regeneration of a standard cation resin. The result is a large excess of phosphoric acid in the potassium phosphate/phosphoric acid solution. No references could be found in the literature for describing regeneration of a cation resin loaded with potassium for producing potassium phosphate.
The most common method used by industry for removing potassium from agricultural by-products is crystallization. Only part of the potassium can, however, be removed and about 3-6 g/100 g based on dry matter remains. The other technologies are more efficient and are able to reduce the level to 1.5 g/100 g dry substance, or even lower. Unfortunately electrodialysis and chromatography are, however, very sensitive to fouling by organic compounds and are therefore less suitable for removal of potassium from agricultural by-products on an industrial scale. Fouling can also be a serious problem in ion exchange technology, but there are several ways to prevent or reduce the effect of fouling during operation.
In the crystallization processes described, the chemical composition of the potassium-rich fraction is mainly potassium sulphate or syngenite (potassium-calcium sulphate) contaminated with organic compounds. For chromatography and electrodialysis, the main product is a mixture of organic potassium salts. For the ion exchange process, the chemical composition of the potassium-rich solution is mainly determined by the acid used for regeneration:
In all the publications relating to this subject, little or no attention is given to further downstream processing of the potassium-rich fraction to increase its purity and value The production of a high-purity potassium salt from an agricultural or fermentation by-product was considered impossible due to the large amount of organic impurities present. Further, none of the references refer to the production of high-purity potassium nitrate or potassium phosphate from an agricultural or fermentation by-product.